1. Field of the Invention
The present invention relates to piezoelectric ceramics and to surface wave devices using the same, and more particularly, relates to a low loss piezoelectric ceramic for use in high frequency filters and in oscillators, specifically, for use in surface wave devices, and to a surface wave device using the same.
2. Description of the Related Art
Filters and oscillators using piezoelectric ceramics have been used for various kinds of electric/electronic products, such as communication apparatuses and audio-visual apparatuses. Recently, filters and oscillators using piezoelectric ceramics are being used at higher frequency ranges, for example, filters and oscillators, which use bulk waves, are applicable in practice to ranges of approximately few tens of MHZ by utilizing a shear vibration or a third harmonic thickness vibration. In a range of approximately 60 MHZ or more in which filters and oscillators using bulk waves are difficult to produce, filters and oscillators using surface waves have been used.
The surface wave devices using surface waves, for example, filters and oscillators, are devices exciting and propagating surface waves by supplying electric signals to electrodes thereof, in which at least one pair of electrodes each having at least one finger and being arranged so as to interdigitate with each other is disposed on a substrate having piezoelectric characteristics. As surface waves used for the surface wave devices, Rayleigh waves are most commonly used, and an SH wave (a horizontally-polarized shear wave), such as a BGS wave (the Bleustein-Gulyaev-Shimizu wave) and a Love wave, is also used which is a shear wave whose displacement is perpendicular to the propagating direction and the component is parallel to the surface of the substrate. The resonant frequencies and electrical and mechanical characteristics of the surface wave devices greatly depend on the characteristics of materials used for the piezoelectric substrates, as is the case with other piezoelectric devices, and are almost determined by the structures of comb electrodes each having at least one finger and being arranged so as to interdigitate with each other (IDT; Interdigital Transducer). Accordingly, improvement of the characteristics of the piezoelectric substrate is an effective technique for improving characteristics of the surface wave device.
As examples of surface wave devices using piezoelectric ceramics, proposals are disclosed in, for example, Japanese Unexamined Patent Application Publications No. 5-145,368, No. 5-145,369, No. 5-145,370 and No. 5-183,376, in which critical characteristics of materials for use in the surface wave devices are described. In addition, various proposals for improving characteristics of the surface wave devices in view of the compositions of piezoelectric ceramics were subsequently disclosed in, for example, Japanese Unexamined Patent Application Publications No. 5-275,967, No. 5-327,397, No. 8-310,862 and No. 9-93,078.
In the surface wave devices using the piezoelectric ceramics as piezoelectric substrates, there is a problem in that the loss at the high frequency ranges is large. Accordingly, single crystal materials, such as LiNbO3, LiTaO3 and quartz are primarily used for surface wave devices intended for use at high frequency ranges of not less than approximately 80 MHZ. The losses of the piezoelectric ceramics are larger than those of the single crystal materials, and the reason for this is believed to be that the mechanical quality factor Qm is small, the surface condition is degraded during micro-fabrication (poor workability in micro-fabrication), pores are generated, and so on. In addition, some of the surface wave devices using SH waves exploit the reflection at the edge surface thereof, and in these devices, the condition of the edge surface reflecting the surface waves influences the loss thereof. Accordingly, one of the reasons the device using the piezoelectric ceramic has a larger loss is believed to be poor workability in micro-fabrication of the edge surface reflecting the surface wave.
As methods for solving the problems of the surface wave devices using the piezoelectric ceramics, the critical characteristics of materials for use in the surface wave devices are disclosed in, for example, Japanese Unexamined Patent Application Publications No. 5-145,368, No. 5-145,369, No. 5-145,370 and No. 5-183,376 as described above. In addition, in Japanese Unexamined Patent Application Publications No. 5-275,967, No. 5-327,397, No. 8-310,862, No. 9-93,078, and so on, improvements in the loss and the heat stability of the piezoelectric ceramics are disclosed. However, when a surface wave device to be used in a range of 80 MHZ or more is formed according to Japanese Unexamined Patent Application Publications No. 5-145,368, No. 5-145,369, No. 5-145,370, No. 5-183,376, No. 5-275,967 and No. 5-327,397, the ratio of the anti-resonant impedance Za to the resonant impedance Zr (i.e., Za/Zr) of the surface wave device is rapidly decreased at approximately 80 MHZ, whereby the device thus formed is difficult to use in practice. In a filter to be used in a narrower band range according to Japanese Unexamined Patent Application Publications No. 8-310,862 and No. 9-93,078, the electromechanical coefficient kBGS is too large and the Za/Zr value is insufficient at 80 MHZ and more, and hence, there is a problem in practical use. The reasons for the decrease in the Za/Zr of the piezoelectric ceramic are believed to be that, in particular, the density of the sintered piezoelectric ceramic is low due to the existence of pores, the stability at high frequency ranges and the workability in micro-fabrication are inferior, and so on.
Accordingly, it is an object of the present invention to provide a piezoelectric ceramic which has significantly low loss and superior workability in micro-fabrication, and to provide a surface wave device using the same.
A piezoelectric ceramic according to the present invention comprises at least lead (Pb), manganese (Mn), niobium (Nb), titanium (Ti) and zirconium (Zr) as primary metal components, wherein, when the composition of the primary components is represented by the formula Pbx{(MnaNbb)yTizZr(1xe2x88x92yxe2x88x92z)}O3, the x, y, z, a, and b are, on a molar basis, such that 0.95xe2x89xa6xxe2x89xa60.995, 0.055xe2x89xa6yxe2x89xa60.10, 0.40xe2x89xa6zxe2x89xa60.55, 2.01xe2x89xa6b/axe2x89xa62.40 and a+b=1, and the average grain diameter of the sintered piezoelectric ceramic is about 2 xcexcm or less.
The piezoelectric ceramic according to the present invention preferably comprises not more than about 0.05 wt % SiO2 contained in the major components.
In the piezoelectric ceramic according to the present invention, the z is preferably about 0.47 to 0.55, and the crystal system of the composition is preferably a tetragonal system.
In addition, in the piezoelectric ceramic according to the present invention, not more than about 5 mol % lead may be substituted with one of strontium (Sr), barium (Ca) and calcium (Ca).
Furthermore, a surface wave device of the present invention is formed of the piezoelectric ceramic according to the present invention.
A Pb {(Mn⅓Nb⅔)TiZr}O3-based material is one of the materials having the lowest loss among PZT-based piezoelectric ceramics, as disclosed in xe2x80x9cPiezoelectric Ceramic Materialsxe2x80x9d P128, 1973, published by Gakken-sha. In order to significantly decrease the average grain diameter of the sintered material described above, in the present invention, it was discovered that a dense piezoelectric ceramic could be obtained which had significantly finer grains and lower loss at high frequency ranges, by a composition containing Nb above the known ratio of Nb to Mn in the conventional composition. In addition, when the amount of Pb is decreased below the stoichiometric content thereof, no foreign phase, such as a pyrochlore phase (Pb2Nb2O7), exists in the sintered material, and hence, a piezoelectric ceramic having an even lower loss can be obtained. When the crystal system of the piezoelectric ceramic is the tetragonal system, the coercive electric field is further improved and the stability of polarization is increased, and hence, an even lower loss at high frequency ranges can be achieved. In addition, when the content of SiO2 in the primary components is about 0.05 wt % or less, the fracture mode of the piezoelectric ceramic is intergranular fracture mode or intergranular-transgranular fracture mode, and hence, serious damage to the piezoelectric ceramic during fabrication can be avoided.
Furthermore, when the piezoelectric ceramic of the present invention is used for a surface wave device, workability in micro-fabrication is superior due to the fine grains thereof, and in particular, when the average grain diameter of the sintered material is about 2 xcexcm or less, the loss at high frequency ranges can be significantly reduced.
The objects described above, other objects, features, and advantages of the present invention will be more apparent from the following detailed description of preferred embodiments thereof with reference to the accompanying drawings.